Faults in electric power systems occur for a variety of reasons, such as trees falling across power lines, transformer failure, shorts in load circuits, and so forth. Once the line section experiencing the fault has been identified, sectionalizing switches are used to isolate the faulted line segment in order to bring the non-faulted sections back into service. Isolating the fault requires identification of the location of the fault with respect to a number of sectionalizing switches that are used to isolate faulted line sections. For a radial distribution circuit, the direction from the substation toward the load is referred to as the forward direction and the direction back toward the substation is referred to as the reverse direction. For a loop fed transmission or distribution circuit, however, the direction along a power line (forward or reverse) is defined as a matter of convention. Sectionalizing switches or circuit breakers and associated monitoring equipment (e.g., voltage and current measuring devices) are typically located at the substations and at major tap points along the power line where transmission or distribution lines “T” from the main power line to pick up loads. When a fault occurs, one or two sectionalizing switches are typically operated to isolate the faulted line segment so that the non-faulted line segments can remain in service.
At present, fault directionality can only be determined at substations where three phase voltage measurements are available. Directional fault identification is not available at tap points away from the substation due to the unavailability of the three phase voltage measurements and the high cost of obtaining those measurement away from the substations. Tap points are therefore “blind spots” for directional fault identification. As a result, over current and voltage techniques are typically used to detect the existence of faults away from the substations and systematic switch closing is typically used to determine the faulted line sections. This is conventionally accomplished by opening all of the sectionalizing switches on a faulted circuit and then systematically closing the switches until the faulted line section has been identified.
Although identification of the faulted line section could be facilitated by determining the direction of the fault at the location of each sectionalizing switch, this is not presently feasible due to the high cost of directional fault detection equipment. More specifically, the faulted line segment could potentially be identified by determining that the fault is forward from one sectionalizing switch and reverse from the next sectionalizing switch along the line. In addition, a tapped line could potentially be identified as the faulted line segment when the change in the direction of the fault occurs at the tap point (i.e., the fault is toward the tap point from both directions along the power line serving the tap point).
However, determining the directionality of a fault on a three phase power line is conventionally accomplished with a voltage monitor and current monitor for each phase, requiring three voltage monitors and three current monitors at each monitoring station. Current monitors are relatively inexpensive whereas the voltage monitors can be a major expense. Although three phase voltage measurements are typically available in substations, they are not generally available at tap points along the power line. The extent of outages could be reduced, by locating directional fault detection equipment at the major tap points along the power line, not just at the substations. But this is typically not economically feasible due to the high cost of installing three voltage monitors at each tap point.
There is, therefore, a continuing need for improved and more cost effective electric power fault isolation systems. There is, in particular, a need for a directional sectionalizing system for a three phase power line that does not require three voltage measurements to determine the directionality of a high impedance fault.